Non-contacting face seal assemblies are usually applied to high-speed, high-pressure rotating equipment where the use of ordinary mechanical face seal assemblies with face contact would result in excessive generation of heat and wear. Non-contacting operation avoids this undesirable face contact when the shaft is rotating above a certain minimum speed, which is often called a lift-off speed.
As with ordinary contacting-type mechanical seal assemblies, a non-contacting face seal assembly consists of two sealing rings, each of which being provided with a very precisely finished sealing surface or face. These surfaces are perpendicular to and concentric with the axis of shaft rotation. Both rings are positioned adjacent to each other with the sealing surfaces in contact at a condition of zero pressure differential and zero speed of rotation. One of the rings is normally fixed to the rotatable shaft, and the other is located within the seal housing structure and allowed to move axially. To enable axial movement of this sealing ring and yet prevent leakage of the sealed fluid, a static sealing element is placed between this ring and the housing. This static sealing element must permit some sliding motion while under pressure, and therefore a top quality O-ring is normally selected for that duty. This O-ring is often called the secondary seal.
To achieve non-contacting operation of the seal assembly, one of the two sealing surfaces in contact is usually provided with shallow surface recesses, which act to generate pressure fields that force the two sealing surfaces apart. When the magnitude of the forces resulting from these pressure fields is large enough to overcome the forces that urge the seal faces closed, the sealing surfaces will separate and form a clearance, resulting in non-contacting operation. The character of the separation forces is such that their magnitude decreases with the increase of face separation. Opposing or closing forces, on the other hand, depend on sealed pressure level and as such are independent of face separation. They result from the sealed pressure and the spring arrangement acting on the back surface of the axially movable sealing ring. Since the separation or opening force depends on the separation distance between sealing surfaces, during the operation of the seal or on imposition of sufficient pressure differential, equilibrium separation between both surfaces will establish itself. This occurs when closing and opening forces are in equilibrium and equal to each other. Equilibrium separation constantly changes within the range of gaps. The goal is to have the low limit of this range above zero to prevent face contact. Another goal is to make this range as narrow as possible, because on its high end the separation between the faces will lead to high seal leakage. Since non-contacting seals operate by definition with a clearance between sealing surfaces, their leakage will be higher than that of a contacting seal of similar geometry. Yet, the absence of contact will mean near-zero wear on the sealing surfaces and also a relatively low amount of heat generated between them. It is this low generated heat and a lack of wear that enables the application of non-contacting seal assemblies (commonly referred to as dry gas seals) to high-speed turbo machinery, where the sealed fluid is gas. Turbo compressors are used to compress this fluid and since gas has a relatively low mass, they normally operate at very high speeds and with a number of compression stages in series.
1. Field of the Invention
During a typical period of operation, a turbo compressor is started and the power unit starts the shaft rotating. At the initial warm-up stage of operation, shaft speeds may be quite low. Typically, oil is used to support the shaft at its two radial bearings and one thrust bearing. Oil warms up in oil pumps and also accepts shear heat from compressor bearings. The oil together with process fluid turbulence and compression in turn warm-up the compressor. Once the full operating speed is reached the compressor reaches in time some elevated equilibrium temperature. On shutdown, shaft rotation stops and the compressor begins to cool down. In this situation, various components of the compressor cool down at different rates and, importantly, the shaft contracts with decreasing temperature at a different rate than the compressor casing. The net result of this at the seal assembly is the axial creeping motion of the shaft and the seal parts fixed to it, which may move the rotatable sealing face away from the stationary sealing face. With often only a spring load behind the stationary sealing ring, the stationary sealing face may not be able to follow the retracting rotatable face, if the above-mentioned secondary seal has too much friction. The term used often in the industry for this phenomenon is "seal face hang-up". In such case very high leakage of process fluid may be observed the next time the compressor is restarted and often in such cases the seal assembly must be removed and replaced at a considerable cost in time and lost production. Initial reason for the high secondary seal friction is the O-ring manufacturing non-uniformity. This non-uniformity is subsequently aggravated by gradual O-ring swell in the presence of oils, other liquids and due to ingestion of sealed gas and by the accumulation of dirt in the vicinity of the O-ring. Consequently, if such a secondary seal is placed into radially un-yielding space, it exerts increasing radial loads on contact surfaces resulting in increasing friction. Eventually the friction becomes so high that the seal hangs-up open.
2. Description of Prior Art
Prior art per U.S. Pat. No. 3,245,692 FIG. 1 teaches a secondary seal arrangement, where axial wave spring biases an O-ring in axial direction against a stationary seal face and a garter spring biases it inwardly at an angle towards the cylindrical housing extension. Unlike with present invention, two spring means are employed to do the biasing of the O-ring. Further, several solutions of the seal face hang-up appeared recently, such as U.S. Pat. Nos. 5,370,403 and 5,560,622 as well as 5,639,097. The improvement these items of prior art provide is a compliant spring element, positioned at the outer diameter of and cooperating with the secondary O-ring. The requirement to squeeze the secondary O-ring into a uniform radial gap with the consequence of high friction forces is thus eliminated, as compliant spring element will exert radial force, which is more or less independent of the O-ring non-uniformities. Subject prior art thus requires an additional spring element to provide this elastic radial load around the elastomer sealing O-ring. Again, unlike with present invention, two spring means of biasing are employed.